Make enum discriminants i64 instead of the host int.
GC static_size_of_enum, which was unused
Add parser for `#[repr(...)]`; nothing uses it yet.

FIXME: document it.
Fix vec::each* return values
Lots of stuff that doesn't work yet
Export attributes on enums
Move repr hint extraction into ty
Add some more missing annotations
Lint non-FFI-safe enums (and temporarily deny them by default)